Thermionic cathode and method of making the same



June 16, 1925. 1,542,385

J. E HARRIS THERMIONIC CATHODE AND METHOD OF MAKING THE SAME Filed001;;12, 1920 /m/en for: James E. Han/ls.

y WAIT Patented June 16 1925.

UNITED STATES PATENTOFFICE.

JAMES E. HARRIS, 01 EAST OR ANGE, NEW JERSEY, ASSIGNOR TO WESTERNELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEWYORK.

THERMIONIC CATHODE AND METHOD OF MAKING THE SAME.

Application filed October 12, 1920. Serial No. 416,386.

To all whom it may concern:

' Be it known that I, JAMES E. Harms, a citizen of the United States,residin at East Orange, in the county of Essex, btate 5 of New Jersey,have invented new and useful Improvements in Thermionic Cathodes andMethods of Making the Same, of which the following is a full, clear,concise, and exact description.

This invention relates to electron emitting cathodes such -as are usedin vacuum tubes, and in a broad aspect relates to alloys. Its object isto provide an electron emitting cathode having high electron emittingactivit The invention comprises such a catho e having a core and a.thermionically acti ve coating therefor, the core containing a'refractory and preferably malleable metal of low volatility such asplatinum or palladium, and a metal such as nickel or cobalt.

Nickel and cobalt for example, are characterized by the fact that uponbeing baked with an oxide of an alkaline earth metal in air they reactwith oxygen and the oxide much more readily than does platinum to form acompound which breaks down to the original constituents when heated in avacuum. After about sixteen hours of operation in a vacuum tube, acathode filament having the platinum-nickel core and a coating thereforcomprising oxides of alkaline earth metals has an-electron emittingactivity considerably higher than that of a filament consisting of aplatinum core and a coating therefor of said oxides. A

The invention also comprises a method of making the cathode by coatingthe platinumnickel core with oxides of alkaline earth metals, andfurther, comprises the core and an alloy containing the metals used forthe core. The preferable coating materials and the preferable manner ofapplying them are set forth in detail below.

In the accompanying drawing, Fig. 1 represents in enlarged form aportion of a filamentary conductor which is suitable for use as anelectron emitting cathode when constituted in accordance with thisinvention. Fig. 2 illustrates the use of such a filament in a thermionicrepeater of the audion type.

A cathode is deslgnated 2 in each figure and is shown as a twistedribbon or filament having, for example, a width of about .3 of amillimeter and the thickness of about .05 ofa millimeter.

Filaments comprising metallic cores coated with oxides of alkaline earthmetals are a carbonate of an alkaline earth metal is baked in air (.asdescribed below) nickel oxide is formed and an oxide of the alkalineearth metal is formed, and then a compound containing nickel and theoxide of the alkaline earth metal is formed. In the subsequent bakingoperation (as described below) during the pumping out of the vacuum tubethe compound breaks down yielding oxygen and nickel and the oxide of thealkaline earth metal so that the coating then left on the core comprisesthe alkaline earth oxide in intimate contact with finely divided nickel.When the platinum-nickel core coated with the made of the alkaline earthmetal is baked in air the nickel oxidizes much more readily than theplatinum and although reaction takes place between the coating materialand nickel very little reaction takes place between the platinum and thecoating materials. After the cathode filament comprising theplatinumnickel core and the coating therefor -containing oxides ofalkaline earth metals in intimate contact with finely divided nickel hasbeen held at its normal operating temperature, in the vacuum tube, forabout sixteen hours, it has an electron emitting activity about doublethat of a filament consistin of a platinum core and a coating there orof oxides of alkaline earth metals in intimate contact with finelydivided platinum.

Proportions of 95% latinum and 5% nickel give the most satis actoryalloy when due consideration is given to the malleability desirable inorder that the alloy may be worked into wire, the degree of activity ofthe finished cathode and the burn out life and especially the activitylife of the cathode. Probably the low limit of the platinum content andalso of the nickel content for a satisfactory core is determinedprincipally by the rate of volatilization or burning out of the cathodewhen in use in vacuum tubes.

Pure platinum burns out more rapidly than platinum alloyed with smallquantities of nickel; but nickel is very volatile and too much nickelialloyed with the platinum will decrease the burn out life of thecathode. When the nickel content of the core is increased to more thanabout 15%, the rate of burning out of the cathode becomes higher thanthat of a platinum core filament; and when the nickel content of thecore is decreased toless than 2%, the rate of burning out of the cathodebecomes of the order of that of a platinum core filament. Moreover, anickel content of about 2% (or more) of the core material is desirablein order to make the coating material adhere strongly to the core.However, the scope of my invention is not to be regarded as limited toany stated proportions of constituents, since primarily the object ofthe invention is to produce a cathode having a high electronemittingactivity.

A preferable method of making the alloy core is the invention of Mr.Howard T. Reeve and is claimed in an application in his name, Serial No.416,464, filed October 12, 1920, methods of making cores for cathodes ofvacuum tubes, assigned to the assignee of this application. The methodwill now be set forth in detail in connection with the description ofthe process of making the cathode of this invention.

The constituents of which the alloy is to consist are thoroughly mixedin the form of finely powdered metals in the desired proportions, forexample 95% platinum and 5% nickel. The mixture is pressed in. a steeldie to form a coherent ingot of convenient size and shape. For an ingotweighing about 25 grams, a convenient form is a, cylinder of an inch indiameter. The pressure used on the die may be exerted manually, butpreferably a pressure of at least several tons per square inch, such ascan be conveniently obtained from a hydraulic press or the like, shouldbe used. The ingot is next fused or sintered, in a vacuum or in an inertas. This fusing or sintering may be done by puttin the ingot in a smallrefractory crucible of alundum, magnesia or the like, the crucible beingplaced in a vacuum furnace of any suitable type. A type which has beenused with good results has an evacuated space connect ed with any goodoil pump capable of producing a vacuum of .001 mm. of mercury, thisspace containing a helical heating coil like, w

molybdenum, or the ich surrounds the crucible. is heated by the passageof a large, low voltage -electric current from the secondary winding ofa step-down transformer. The vacuum chamber preferably has a transparenttop so that the temperature of the ingot may be observed with an opticalpyrometer. Since the melting point o-f nickel is about 1450 C. and thatof platinum is is about 17 50 (3., the temperature used for sintering orfusing the ingot is in the neighborhood of 1500.C. to 1800 C. Theduration of the fusing or sintering operation should be as short aspossible, in order that the amount of nickel vaporized shall be as smallas possible. The duration will depend upon the size of the ingot. With a25 gram ingot in the form of a cylinder A; of an inch in diameter,fusing or sintering in the crucible would be for only a few minutes.

Another way in which the sintering temperature may be obtained is byclamping the ingot between two electrodes in a belljar, exhausting theair from the jar, and passing a heating current through the ingot,increasing the current until a sintering temperature of about 1500 C.,as observed with an optical pyrometer for example, is reached, and thencutting the current ofl'. Where the ingot is heated in this manner, theduration of the sintering will be only momentary and will be only fromabout a quarter to a tenth as long as where the ingot is fusedorsintered in a crucible in a furnace as referred to above.

The pressures maintained by the pump during the fusin or sintering havebeen from .01 to .001 o a millimeter of mercury, though probably avacuumof only a few millimeters would 'sufiice. However, the higher thevacuum, the better will be the results, since the sintering in a vacuumor in an inert gas prevents oxidation of the nickel. Of course, as isindicated above, the fusing or sintering may be done in an inductionfurnace or in any suitable vacuum furnace.

When the ingot has -been fused or sintered, it is forged to get it intothe form of a rod and 'then the rod is swaged into wire, whichis drawnthrough diamond dies or the like to the required size, all of thisforging, swaging and drawing being cold working. Hot Working is notadvisable for if the platinum nickel alloy were worked hot the nickelwould be oxidized and as a consequence the material or work, wouldbecome very brittle. The material is annealed at frequent intervalsduring all of this forging, swaging and drawing. Great care of graphite,tungsten,

The coil must be taken that the working at any stage of the working,

come so brittle that it will. crack. The method of annealing depends onthe stage thatis, on the dimensions, especially the ngth of the work. Ingeneral, the larger the diameter, the higher the should be between about1100 C. and the fusing temperature, which latter temperature, is in theneighborhood of 170 0 C. Regarding the .duration of the heating forannealing, it is only necessary to bringthe work up to the annealingtemperature and then let it 'cool oil. The method of heating forannealing should preferably besuch as will guard against oxidation ofthe nickel. When the working is in the first sta es, for instance whenthe work is in the orm of an ingot of the size referred to above, theWork may well be heated to the annealing temperature in the oxy-hydrogenflame, and preferably cooled in a reducing, hydrogen flame (the supplyof gas from the oxygen tankbeing shut off) until the ingot is below redheat, when it is withdrawn from the flame and quenched in water. Whenthe work has increased considerabl in length,

the method preferred consists in clampingthe work between electrodes, ina vacuum, and passing an electric current through it to give therequired temperature, which as is indicated above, is in the lower partof the range from 1100 C. to 1600 C. In any case, if the work becomesoxidized, the oxide can be broken down by heating the work in a vacuumfor a few minutes at a temperature of about 1000 C.

The wire made from the ingot as described above is suitable for use as acore for coated cathodes of the type referred to above. However, thewire is preferably cold rolled into ribbon form having, for example, awidth of .3 of a millimeter and a thickness of .05 of a millimeter andthe ribbon coiled and pulled out into the twisted filamentary formcommon for thermionic cathodes, and

shown, for example, in United States patent to Nicolson and Hull,referred to above.

A filament having high activity can be made by coating the coredescribed above with an oxide of any alkaline earth metal,

by applying a coating of the carbonate or hydroxide of the metal to thecore, heating the coated core in air for about 5 to 20 minate to theplatinum-nickel core by a method somewhat similar to that disclosed fora plying such coatings to a platinum core in plication. annealingtemperature. The temperature the application of Carl D. Hooker, SerialNo. 252,689, filed September 5, 1918, process of manufacturing electronemitting cathodes, assi ned to the assignee of this apcoats of thecarbonates should be applied. The coating material is embodied or:suspended-in a solid-carrier which may be of waxy material such asparaflin, by stirring together the desired quantity of strontiumcarbonate and barium carbonate into the same batch of molten parafiin,until the mixture has cooled and then forming the mix ture into rods orpieces of other convenient shape. Preferably, the relative amounts ofthe ingredients are 2 grams of barium carbonate, 7 grams of strontiumcarbonate, and 100 grams of paraflin. To apply each coat, the solid rodcontaining thecarbonates is passed along the length of the core, thecore being heated in the usual way (that is,

.by passing a current through it) to a temperature sufficiently high tomelt the paraf fin, so that the coating material will be deposited onthe core. After the application of each coating, current should bepassed through the strip to flash it momentarily to aired heat whichserves to burn oil the parafiin, reducing the alkaline earth compound toan oxide which adheres closely to the filament. During the coatingprocess, the filament may be ield by any suitable supports. After thelast coat has been applied. to the gore, the filament is baked in airfor about 5 to 20 minutes at a temperature of about 1200 C. The filamentis, of course, later heated in a vacuum for several minutes at atemperature of about 1000 (1, during the pumping of the vacuum tube.After the filament has had its normal operating current passed throughit for about 16 hours. in the vacuum tube, ithas an electron-emittingactivity about twice as great as that of a filament made by similarlycoating a platinum core.

One of the important advantages of the alloy of this invention is thatit may consist of pure metals such as platinum and nickel so that itscomposition can be controlled by chemical analysis, whereas with theplatinum core now in common use containing platinum mixed with varyingamounts of iridium and rhodium, a satisfactory analysis is a mostdiflicult matter.

The nickel in the platinum-nickel core produces a desirable increase -inthe ohmic resistance of the core, as do the iridium and rhodium in theplatinum core now in common use containing those metals. However, theiridium and rhodium may of course also be incorporated in the mixture ofpowdered platinum and nickel used in making the core of this invention,for further increasing the resistance, without departing from theinvention.

referably, a total of about 16' nickel oxidizes much more rapidly thanplatinum.

. It should be understood that where platinum and nickel are referred toherein, without mention of their equivalents, such refer ences are to beregarded as also including equivalents of the platinum, for instance,palladium, osmium, rhodium, iridium and ruthenium, or equivalents of thenickel, for instance, cobalt.

What is claimed is:

1. An" electron emitting n'iaterial, comprising an alloy containing arefractory metal and a metal in the nickel-cobalt group, said alloyhaving the property of yielding when coated with an oxide of an alkalineearth metal and baked in air and then in a vacuum, a coating having anelec tron-emitting activity considerably greater than that of a coatingmade by similarly treating platinum.

2. A cathode containing a core formed from an alloy comprising a metalof the platinum group and a metal which upon be ing baked with an oxideof an alkaline earth metal in air reacts with oxygen and the oxide ofthe alkaline earth metal to form a compound that breaks down to theoriginal constituents when heated in a vacuum, said alloy having thepropert of yielding, when coated with an oxide 0 an alkaline earth metaland baked in air and then in a vacuum, a coating having anelectron-emitting activity considerably greater than that of a coatingmade by similarly treating platinum.

An electron-emitting cathode comprising an alloy containing platinum andat least 2 per cent of nickel, and a coating containing thcrn'lionicallyactive material, said cathode having an electronemitting activ ityconsiderably greater than that of a platinum filament coated withalkaline earth metal oxides.

i. An electron-emitting cathode comprising an alloy containingapproximately 95% platinum and 5% nickel.

5. A cathode containing a core formed from an alloy comprising platinumand a metal of the nickel-cobalt grou which u on being baked with anoxide 0? an alka ine earth metal in air reacts with oxygen and the oxideof the alkaline earth metal to tron-emitting form a compound whichbreaks down to the original constituents when heated in a vacuum, saidalloy having the pro erty of yielding, when coated with an oxi e of analkaline earth metal and baked in air and then in a vacuum, a'coatinghaving an electhan that o a coating made by similarly treating platinum.a

6. An electron-emitting element having a core comprisin a refractorymetal and a metal of the nic rel-cobalt group which upon being bakedwith an oxide of an alkaline earth metal in air, reacts with oxygen andthe oxide of the alkaline earth metal to a materially greater extentthan said refractory metal to form a compound which breaks down to theoriginal constituents on being heated in a vacuum, said element having acoatin for the core comprising an oxide of an a kaline earth metal.

7. The process of making an electronemitting cathode which comprisesbaking a filament containing platinum and nickel with an oxide of analkaline earth metal in the presence of oxygen.

8. The process of making an electronemitting cathode which includesbaking a core containing platinum and nickel with an alkaline earthmetal in the presence of oxygen for a short period to form a compoundcontaining nickel and said alkaline earth metal and later baking thecore and the compound in the absence of oxidizing conditions to causesaid compound to break down into nickel, oxygen and an oxide of saidalkaline earth metal.

9. The process of making an electronemitting cathode which includesbaking a core containing platinum and nickel with oxides of barium andstrontium in the presence of oxygen for about five to twenty minutes ata temperature of about 1200- 0., and later baking the coated core in theabsence of oxidizing conditions at a temperature of about 1000 C. for aperiod of several minutes.

10. The method of making an electronemitting cathode which comprisescoating a core containing platinum and nickel with material comprisingan alkaline earth metal and baking the coated core in the presence ofoxygen about five to twenty minutes to cause reaction between the coreand .the coating material.

11. An electron-emitting cathode containing platinum, nickel, bariumoxide and strontium oxide.

12. An electron-emitting cathode comprising an alloy of platinum andnickel and also comprising an alkaline earth metal.

13. An electron-emitting cathode comprising a core containing latinumand nickel and a coating contaimng nickel.

14. An electron emitting cathode comactivity considerably greaterplatinum and nickel and a coating containing nickel, barium andstrontium.

18. An electron-emitting cathode comprising a core which volatizes onlyat a high temperature and a coating containing nickel and an alkalineearth metal: 19. An electron-emitting cathode comprising an alloycontaining a metal of the platinum group and a metal of the nickelcobaltgroup, said alloy being less volatile than the second mentioned metal,and said cathode having a considerably higher electron-emitting activitythan a platinum filament coated with oxides of alkaline earth metals.

20. An alloy comprising a metal of the platinum group and a metal whichupon being baked with an oxide of an alkaline earth metal in air reactswith oxygen and said oxide much more readily than does platinum to forma compound that breaks down to the original constituents when heated ina vacuum.

21. An alloy comprising a metal of the platinum group and a metal whichupon being baked with an oxide of an alkaline earth metal in air reactswith oxygen and said oxide to form a compoundmuch more readily than doesplatinum.

2-2. An electron-emitting cathode core comprising platinum and a metalwhich upon being baked with an oxide of an alkaline earth metal inair-reacts with oxygen and said oxide much more readily than doesplatinum, to form a compound that breaks down to the originalconstituents when heated in a vacuum. 1

23. An electron emitting cathode comprising an alloy containing a metalof the platinum group and a metal more volatile than said first metal,said alloy being coated with thermionically active material containingsaid more volatile metal.

24. An electron emitting cathode comprising an alloy containing a metalof the platinum group and a metal more volatile than said first metal,said alloy being coated with oxides of alkaline earth metals.

25. An electron emitting material comprising a core, and a. coatingcontaining an oxide of alkaline earth metal baked thereon, said corecomprising a refractory metal alloyed with a metal of the nickel-cobaltgroup capable of forming an unstable com pound by reaction with saidcoating, said material also containing a product formed by reactionbetween said second mentidned metal and said coating, and having anelectron emissivity materially greater than platinumsimilarly coatedand. treated.

26. An electron emitting material comprising an alloy core and a coatingcontaining an oxide of alkaline earth metals baked thereon, said alloycore comprising a refractory metal alloyed with a secondmetal, such asnickel, capable of forming an unstable compound by reaction with saidoxide of alkaline earth metal, said material also containing said secondmetal in finely divided form in intimate contact with said oxide.

, 27. An electron emitting element comprising a core having a coatingcontaining an oxide of alkaline earth metal, said coating comprisingalso a metal of the nickel cobalt group.

28. An electron emitting element comprising a core having a coatingcontaining an oxide of an alkaline earth metal, said core containing inpreponderating amount a refractory metal which remains substantiallyinert toward oxides of alkaline earth metals at the operatingtemperature, said coating containing also a metal of the nickelcobaltgroup.

'29. An electron emitting element comprising a core of a refractorymetal alloyed with a lesser amount of a metal, such as nickel, whichvolatilizes at a relatively lower" temperature than said refractorymetal, and a coating containing an oxide of an alkaline earth metalapplied to said core.e; rid refractory metal being substantiallyinactive chemically with respect to said oxide.

30. A filament comprising a core containing a metal of the platinumgroup alloyed with at least one of the two metals. nickel and cobalt.and a coating containing strontium and barium oxides,'said filamenthaving a degree of thermionic activity considerably greater than aplatinum filament similarly coated and operated under similarconditions.

31. An electron emitting cathode compri ing an alloy containing platinumand a. maximum of 15 per cent of nickel, and a coating containing a.thermionically active material, said cathode havingan electron emittingactivity considerably greater than that of a platinum filament similarlycoated.

In witness whereof I hereunto subscribe. my name this 9th day of OctoberA. D.,

